1. Field of the Invention
The present invention relates to an imaging system, an image sensor, and a method of controlling the imaging system.
2. Description of the Related Art
An imaging system such as a digital camera sometimes uses an image sensor including a pixel array in which a plurality of pixels are arrayed in a matrix, as shown in FIG. 25. The following technique is proposed to control the charge accumulation operation of pixels on each row of the pixel array.
As a general technique, the start of the charge accumulation operation of pixels on each row of the pixel array is controlled by the pixel reset operation, and the completion of the charge accumulation operation is controlled by the read operation of signals from pixels.
As another technique, the start of the charge accumulation operation of pixels on each row is controlled by the pixel reset operation, and the completion of the charge accumulation operation is controlled by light shielding by a mechanical shutter (see Japanese Patent Laid-Open No. 11-041523), as shown in FIG. 26. According to this technique, the charge accumulation times of pixels on respective rows can be made equal by setting, in accordance with the traveling pattern of the mechanical shutter, the pattern of the timing when the reset operation of pixels on each row is completed.
According to the technique disclosed in Japanese Patent Laid-Open No. 11-041523, the reset operation of pixels on one row starts a predetermined period after the completion of the reset operation of pixels on another row.
For example, at the timing when the Kth row selection period (K; natural number, K=k) starts, a reset signal φ RDk changes to a high level, and the reset operation of pixels on the Kth row starts, as shown in FIG. 27. In the Kth row selection period, the reset signal φ RDk changes to a low level, and the reset operation of pixels on the Kth row terminates. After the reset operation of pixels on the Kth row terminates and the Kth row selection period terminates, the reset operation of pixels on the (K+1)th row starts.
It should be noted that, in FIGS. 25 and 27, φ TRk denotes a transfer signal for turning on a transfer switch QTkj (j=1, 2, 3) to transfer a signal from a photodiode PDkj to a gate of a transistor QAkj, φ RDk denotes a reset signal for resetting the gate of the transistor QAkj, φ CKV1 and φ CKV2 denote scanning clock signals, φ PG denotes a reset control signal, φ RDR denotes a reset potential selection pulse, φ TRR denotes a reset pixel transfer selection pulse, φ RDS denotes a readout potential selection pulse, φ TRS denotes a readout pixel selection pulse, φ STVS denotes a readout start pulse, and φ STVR denotes a start pulse for reset operation.
The time during which the mechanical shutter passes each pixel on one row is shorter than that of a general electronic shutter. When the pattern of the timing of the completion of the reset operation is set in accordance with the traveling pattern of the mechanical shutter, the Kth row selection period needs to be set shorter than that for a general electronic shutter. As a result, the period from the start to completion of the reset operation becomes short.
In this manner, when the reset operation of pixels on one row starts a predetermined period after the reset operation of pixels on another row is completed, the period from the start to completion of the reset operation shortens. In this case, charges accumulated in pixels may not be able to be adequately reset, and an image lag may appear in a photographed image.